In a conventional shift-by-wire system for an automatic transmission of a vehicle, driving of a motor is controlled based on a signal representing a target shift range (gear position) corresponding to a range changeover operation (for example, a parking lock operation) of a vehicle driver to thereby change the range to the target range selected by the driver, for example, from a parking range (P-range) to a non-parking range (non-P-range) or from the non-P-range to the P-range. That is, the motor in the shift-by-wire system is rotated to move a changeover mechanism to thereby change the range to the target range selected by the driver. The changeover mechanism is a range changeover mechanism for realizing a changeover to each of the P-range, neutral range (N-range), reverse range (R-range), and drive range (D-range), or a parking mechanism for switching between the P-range and the non-P-range.
The shift-by-wire system has an electronic controller as exemplarily disclosed in JP 2008-184985A (US 2008/0182717 A1). The controller includes a range changeover control ECU, a motor driving circuit and the like. Moreover, the controller is electrically connected to a range detecting device that outputs a signal representing the target range selected by the range changeover operation of the driver, a motor which is a driving source of the range changeover mechanism, and an output shaft sensor that detects an actual range attained in the vehicle transmission. The motor includes an encoder that outputs a pulse signal (encoder signal) in synchronization with rotation of a rotor of the motor.
The control ECU sets a target rotation angle (a target value of the encoder count value) corresponding to the signal representing the target range output from the range detecting device. The control ECU energizes the motor via the motor driving circuit in accordance with the set target rotation angle to thereby control the driving of the motor. In this case, the control ECU performs feedback-control on the motor so that the count value of the pulse signal output from the encoder stops at a position identical to the target value. In this way, the range is changed to the target range selected by the driver. The control ECU monitors whether the detected actual range is identical to the target range based on the output of the output shaft sensor.
The electronic controller in the shift-by-wire system may be divided to a first electronic controller and a second electronic controller. The first electronic controller includes a shift determination unit that determines a range changeover operation of a driver to determine a target range and outputs a range changeover request to a target range. The second electronic controller includes a motor control unit that controls driving of a motor in accordance with the range changeover request of the shift determination unit to move a changeover mechanism that changes a range to change the range to the target range. According to such a divided configuration, even when the determination content of the range changeover operation or the type of the motor is changed, only one controller need be replaced.
Moreover, according to such a divided configuration, the motor control unit of the second electronic controller can detect an abnormality when the state of the changeover mechanism does not correspond to the target range within a predetermined period after the driving control is started. Similarly, the shift determination unit of the first electronic controller can detect an abnormality when the state of the changeover mechanism acquired from the motor control unit does not correspond to the target range within a predetermined period after the range changeover request is output. Moreover, each of the first and the second electronic controllers may store abnormality information for diagnosing or analyzing the cause of abnormality when an abnormality is detected.
However, when an abnormality occurs in the motor control unit of the second electronic controller, only the shift determination unit of the first electronic controller can detect the abnormality. Thus, although the abnormality information is stored in the first electronic controller, it is not stored in the second electronic controller. It is thus difficult to diagnose the cause of abnormality unless the abnormality information is stored in the second electronic controller including the motor control unit where an abnormality has occurred.